Third generation (3G) wireless communication systems, such as the Universal Mobile Telecommunication System (UMTS), permit interoperability between radio access networks, such as the Global System for Mobile Communications (GSM) and the Universal Terrestrial Radio Access Network (UTRAN). For this reason, universal mobile telecommunication system (UMTS) user equipment (UE), such as mobile radiotelephones, can support multiple radio access technologies (RATs). For example, a UE may support communication using either Wideband Code Division Multiple Access (WCDMA) or GSM RATs, and thereby permit access to wireless communication services via network equipment associated with UTRAN or GSM cells.
For interoperability, the UE implements a process that controls selection and reselection of cells associated with both RAT types, i.e., inter-RAT cell reselection. As an illustration, when a mobile UE is in GSM idle mode, it may perform signal measurements of neighboring UTRAN cells. If the UE determines that one of the neighboring UTRAN cells has a stronger signal level than the serving and neighboring GSM cells, and the signal level satisfies a network-specified threshold, the UE selects the UTRAN cell for communication services.
UEs also implement special processes for handling emergency calls. These processes may utilize the interoperability capabilities of the UE. For example, when a UE is used to make an emergency call, such as a 911 phone call in the US or a 112 phone call in other countries, the UE may redirect the emergency call to a different RAT or frequency. For example, the UE may redirect the emergency call to a different frequency or RAT following an unsuccessful attempt at placing the emergency call using the current frequency. As another example, the UE may switch to a GSM mode to place the emergency call if it is not already operating in the GSM mode. The UE switches to the GSM in this case because the GSM mode provides location information that can be used to locate the UE, which may be helpful in an emergency situation, such as a situation in which the user is unable to communicate the location information to the emergency call handler.
However, the emergency call may be unsuccessful on the serving frequency or after being redirected to a different RAT or frequency because the network is unavailable from the cell and frequency on which the UE is operating. The network may be unavailable for various reasons, such as not being able to determine the location of the UE, congestion on the uplink or downlink channel due to heavy call traffic, and the network shutting down or being unavailable due to maintenance. Each failed attempt at placing the emergency call introduces a delay in placing the emergency call. Delay is undesirable because of the high priority of an emergency call.